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Unlike human arms, which move simply by contracting and relaxing muscles, robotic arms use gears, hydraulics, and other expensive, heavy, power-hungry parts. In spaceowhere robots will carry out sample collection and repairs in many nasa-planned missionsosuch complexity becomes a problem. Heavy items cost more to launch, and power supplies are limited on space probes. All this motivated Yoseph Bar-Cohen to try to make a better robot. And he may have succeeded. He has created artificial muscles that quickly respond to small amounts of electricity by lengthening or bending, almost like the real thing.

Bar-Cohen, a physicist at Caltech's Jet Propulsion Laboratory, has made two different types of artificial muscles. The first works as the muscle in a robotic hand. The muscle is a flexible polymer ribbon constructed from chains of carbon, fluorine, and oxygen molecules. When Bar-Cohen applies an electric charge to one side of the ribbon, charged particles in the polymer get pushed to the far side. The side with more particles lengthens, while the other side shortens. The net result: The ribbon bends. Using four such ribbons, Bar-Cohen has fashioned a gripper that can pick up a rock, as shown in the photos at left.

The other type of polymer muscle consists of thin sheets wrapped into a cigarlike cylinder. These polymers stretch when one side of a sheet is given a positive charge and the other a negative. These charges cause each wrapped sheet to contract toward the center of the cylinder, and this constriction forces the cylinder to expand lengthwise. When the power supply is off, the rope relaxes. These polymers can push, pull, and lift loads.

Bar-Cohen's polymers are cheap, durable, and use just 50 milliwatts of power. But there's a drawback: they're not very strong. He is working on that, but for now he's sticking to minirobots that can lift about a third of an ounce. That should be sufficient if, as Bar-Cohen hopes, his polymers are incorporated into the five-inch rover jpl is building for asteroid exploration. "I see it as an evolution, like the car or the first computer," says Bar-Cohen. "Once you make something like this, there's no end to it."

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